Dry circuit switch



Dec. 11, 1962 J. GREGG 3,06

DRY CIRCUIT SWITCH Filed June 21, 1960' United 3,063,335 DRY (JIRCUITWlTCH .lohn Gregg, Los Angeles, Calif-E, assignor to Litton Systems,Inc., Beverly Pills, Calif. Filed June 211., 1960, Ser. No. 37,747 6(Claims. (Cl. End-166) This invention relates to switching devices andmore particularly to an improved combination of movable and stationaryswitch contacts in a configuration having sp cial application in lowlevel signal or so-called dry circuit applications.

in recent years, the requirements of sophisticated computer and controlsystems have required the transmission of extremely low level signalsfrom remote sensors or transducers into circuits devised to utilizethese signals. Frequently, these signals, in the form of currents in themicroampere range operating under millivolt diiference in potential,represent physical parameters such as tem perature, pressure,acceleration, or velocity which are used in analog or digitalcomputation. Slight variations in the voltage or current, introduced bynon-negligible impedances in the switching circuits, can completelyinvalidate the computations.

Conventional switches, such as are found in commercially availablerelays, are fully adequate at their rated voltage and current levels,nominally amperes and volts, but frequently exhibit an intolerably highimpedance to low level signals in the dry circuit range. For example, arelay switch designed to carry a rated load of 2 ampcres at voltspresents negligible resistive impedance when used in such a circuit.However, the same relay switch used in a low level, dry circuitapplication may exhibit resistive impedances on the order of severalhundred ohms, which may be many times the impedance of the dry circuititself.

In the prior art, special, noble metal relay contacts have been providedsuch as gold alloys, platinum alloys, and others which do not tarnishreadily, and which normally exhibit extremely low contact resistancethroughout the life of the switch. Further, these contacts can besubjected to normal rated volt-ampere loads without damage and withoutadversely affecting their performance under low level conditions.However, such contacts can be expensive compared to the total cost ofthe relay, and, where many hundreds of switch contacts are required in asystem, the increased cost is appreciable.

In order to avoid the high cost of noble metal contacts, attempts havebeen made to plate or flash a noble metal film over a base metal contactto gain the benefits of decreased contact resistance to low levelsignals and lowered incidence of tarnishing films. Although thesecontacts are satisfactory for low-level use exclusively, they arecertain to be rendered useless if subjected to the larger, rated currentor voltage loads. For example, the inadvertent testing of such a relayat its normal rated voltage of, for example, 2 amperes and 15 volts,causes a physical alteration of the surface due to arcing andevaporation of the plating among other things and, although the relay issatisfactory for normal, rated power switching duties, it is ruinedinsofar as further dry circuit use is concerned. More important, shouldsuch a relay be installed in a dry circuit, and, as the result of thefailure of a component in the system, the contacts receive a votlage andcurrent load that is substantially greats than the dry circuit range,then the relay will no longer function satisfactorily under dry circuitconditions and will require replacement.

According to the present invention, relay contacts are so arranged totake advantage of the high conductance and low tarnishing of precious ornoble metals and the cost advantages of base metals to provide a switchcontact which is useful in low level or dry circuit applications andwhich is not damaged by occasional, intentional or inadvertent surges ofcurrent in the normal, rated range. The configuration of the stationarymember relative to the movable member is such that two separate contactsare formed, one of which makes first and breaks last while the othermakes last and breaks first, somewhat in the manner of prior art circuitbreaker devices. It will be clear, however, that the special problemsinvolved with low level circuits prevent the direct adoption ofconventional circuit breakers of the prior art.

in a preferred embodiment of the present invention, the movable blade ofthe switch is bifurcated and is normally biased out of engagement withthe stationary contact elements. One of the two contact ends is termedthe power contact while the other is termed the signal contact. A leverarm connected to a relay armature is used to drive the moving contactagainst its normal spring bias. When the relay is energized and thearmature is pulled, the lever arm moves the entire blade towards thestationary contacts. The blade power contact engages a corre spondingstationary power contact to establish or make the circuit. The lever armcontinues to drive the signal contact portion of the movable blade intoengagement with the corresponding stationary signal contact. When therelay is fully energized, both the signal and power contacts of themovable blade are conductively engaged with their correspondingstationary contacts. The additional travel necessary to engage thesignal contacts after the power contact is made causing a wiping actionon the power contact, and, in the preferred embodiment, sufficientover-travel is provided to cause a wiping of the signal contacts, aswell. As the relay is deenergized the armature is released and thespring bias of the movable blade initially discngages the signalcontacts after which the power contacts separate. The design parametersof the contact assembly are carefully controlled to assure this breakingsequence on deenergization and any arcing or welding that might takeplace is confined solely to the power contacts.

In continuing operation, if all signals are of the low level type, noarcing or heating will ever occur at the power contacts and th reforeboth power and signal contacts are useful for low level or dry circuitsignals. However, should a signal of greater power such as thevolt-ampere signal that is normally within with relays rated voltage andcurrent be applied either due to testing or component failure, anyarcing or welding action is limited to the power contacts. Inasmuch. asa noble metal flashing or plating is adversely affected by the arcing orwelding action at power levels, it follows that the power contacts arerendered unlit for future dry circuit signals but the relay as a unitdoes not lose its suitability, inasmuch as the signal contacts are notaffected and continue to remain useful for low level signalapplications.

In an alternative embodiment of the present invention, a second pair ofstationary contacts are provided for the normal single pole double throwoperation. whereby in one relay state, one set of power and signalcontacts are engaged and in the other state, the other set of contactsare energized. In this arrangement, too, the stationary contacts arestaggered so that the power contacts are always first to make and lastto break and the signal contacts are last to make and first to break.

The expressions low level or dry circuit refer specifically toelectrical currents of less than 106 microamperes and dilferences ofpotential less than millivolts. These limits have been used, forexample, in the published annual proceedings of the National Conferenceon Electromagnetic Relays, sponsored by the National Association ofRelay Manufacturers and Oklahoma State University.

Accordingly, it is an object of the invention to provide a novel relaycontact configuration for reliably connecting low level circuits.

It is an additional object of the invention to provide a highly reliablerelay switch for low level circuits whose operation is unimpaired byvoltage and current overloads.

It is a further object of invention to provide a base metal relay switchcontact having a noble metal flashing or plating that will operatereliably in low level circuits in spite of power loads far in excess ofthe low level circuit range.

It is still another object of the invention to provide a novel relayswitch contact arrangement of increased reliability and lower cost thatis alternatively operable at low level circuit or at power circuitconditions.

It is yet another object of invention to provide a low level or drycircuit switch of high conductance which includes two sets of contactsthat operate in a predetermined make-break sequence.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several embodiments of the invention areillustrated by way of example. it is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only, and are not intended as a definition of the limits ofthe invention.

FIGURE 1 is a perspective view of a switch according to the presentinvention, which includes a movable, bifurcated blade and a pair ofresilient, stationary contacts;

FIGURE 2 is a top view of the switch of FIGURE 1 taken along line 2 inthe direction of the arrow;

FIGURE 3 is an end view of the switch of FIGURE 1 taken along line 3 inthe direction of the arrow;

FIGURE 4 is a perspective view of an alternative embodiment of theswitch of FIGURE 1 which includes a second pair of stationary contactsfor single pole, double throw operation;

, FIGURE 5 is an end view of another switch according to the presentinvention, which includes a movable bifurcated blade and a pair ofrigid, stationary contacts;

FIGURE 6 is an end view of still another configuration of the switch ofthe present invention which includes a non-bifurcated movable blade anda pair of resilient, stationary contacts; and

FIGURE 7 is an end view of yet another embodiment according to thepresent invention which employs contact buttons of different thicknesson a movable bifurcated blade which engage fixed, stationary contacts.

Turning now to the figures, there is shown in EEG- URES l, 2 and 3several views of a preferred embodiment of the present invention whichis illustrated as a single pole, single-throw switch 1%. To simplify thede scription of the present invention, the details of the relay coil andarmature assembly have been omitted from the drawing as being well-knownto those skilled in the art. The switch ill includes a movable blade 12of a rectangular shape. The movable blade 12 is made of a resilientmaterial and is mounted in cantilever fashion to the frame of the relaystructure (not shown). The blade 12 is bifurcated at its free end into apower contact portion M and a signal contact portion in which areco-planar. A permanent resilient bias is imposed to normally bias theblade away from a corresponding stationary contact 18 which is alsodivided into two separately engageable portions, includin a powercontact element 2% and a signal contact element These elements of thestationary contact it; are corna sta ionly connected electrically, andare spaced in gered relation with respect to each other, the powercontact element 2% being positioned in advance of the signal element 22,relative to the movable blade 12. The stationary contact elements arealso of a resilient material to increase the available contact pressureand to assure a wiping action when engaged by the movable blade ll As isclearly seen in the end view (FlG. 3), the movable blade portions arenormally aligned and the stationary elements are stepped to facilitatethe early engagernent and late disengagement of the power contact pair.

A portion of relay armature 24 is shown, from which extends a lever arm26 terminating in a glass bead. The techniques and details of armatureand lever arm construction are well-known in the art and therefore, tosimplify the presentation, only the portion is shown here. The lever arm26 is positioned adjacent the signal contact portion id of the movableblade 12- and, in the normally open position with the relay in thedeenergized state, the arm 25 is spaced away from the blade 12. As shownin the figures, however, the relay is assumed to be partially energizedand the armature is shown after a small amount of travel. With the relayfully energized and the armature 24 at its corresponding pulledposition, both the power and signal contacts are engaged and a slightamount of overtravel of the armature 24 and lever arm 26 has beenpermitted to enable a wiping of the sets of contacts to assure a highlyconductive circuit. The wiping action tends to prevent the formation orretention of tarnish films by a polishing process and otherwise improvesthe conductivity at the area of contact.

All of the contact elements, such as the bifurcated blade 12 and thepower and signal contact elements 20, 22 are preferably made of arelativey inexpensive, resilient, base metal such as phosphor-bronze,copper, or nickel which tend to tarnish and thereby increase sur-= facecontact resistance to low level signals. To prevent this, according totechniques well-known in the art, a noble metal such as gold, platinum,or a noble metal alloy, is flashed or plated on to provide a noble metalcontact surface at a nominal cost. The noble metals exhibit not onlyhigher conductivity, but also a greater resistance to tarnishing and theformation of organic films. Further, due to the great ductility of thenoble metals, rubbing or wiping contact of noble metal surfaces does notresult in an appreciable wearing away of the noble metal, but rather thenoble metal virtually flows into the grain structure of the base metal,impregnating the surface layers beneath to provide at all times, a noblemetal contact surface.

In operation at the extremely low power levels of the so-called drycircuit range, the difference in potential existing between the movingand the stationary portions of the switch is on the order of millivolts.As the power contacts approach each other, conduction is ini tiated inthe circuit approximately at the instant of physical impact. The leverarm 26 continues its travel, making the signal contacts withoutaffecting the character of the transmitted signal. However, due tomalfunction of circuit elements or such inadvertence, as for example, anincorrect test procedure, a diiference in potential exists within thesocalled normal range which includes signals on the order of watts, and,as the power their engagement does not give rise to any destructive.

el ctrical action.

When the lever arm as is released by deenergization of the relay, thenormal bias of the movable blade 12 provides the motion necessary tobreak the contacts,

aoeeese The signal contacts f6, 22 are the first to break, partly because of the relative location of the signal and power contact elements22, and partly because of the additional increased tension on the bladessignal portion 1 5 brought by the lever arm 26 when the circuit is fullymade. At a finite time interval later, the power contacts 14, 20 alsobreak, opening the circuit. Under dry circuit conditions, theinterruption of the circuit produces no deleterious effects upon thecontact elements. In the case of the inadvertent watt-power signalinvolving a several volt difference in potential, however, breaking ofthe circuit also produces arcing and heating at the opposing surfaceswhich causes evaporation and melting of the flashing or plating of thepower contacts.

It may be seen that all damage is confined to the surface of the powercontacts alone, and, even after damage occurs, the switch is stilleffective to transmit low level, dry circuit signals through the signalcontacts. Subsequent operation of the relay at the low signal levelsproduces little or no transmitted signal at the power contacts becauseof the higher surface resistance of the damaged contacting surfaceswhich quickly tarnish and film. However, closure of the signal contactsdoes complete the circuit. At the lower power levels associated with drycircuit operations, the damage caused by initiation of conduction isnegligible. Further, continued imposition of the higher power signalswill not damage the signal contacts since the power contacts remainhighly conductive to signals in the volt-ampere range.

It will be understood by those skilled in the art that commerciallyavailable relays are normally rated in terms of the maximum voltage andcurrent that the contacts can safely and reliably handle. The drycircuit requirements are a relatively recent innovation in the relay artand impose the additional condition that the contact resistance of theswitch at the extremely low power levels encountered, such as, forexample, microampere currents at millivolt potentials, be substantiallyless than 1,000 ohms. This condition can be satisfied either by thecostly expedient of noble metal contacts or by the relativelyinexpensive plating or flashing of base metal contacts, which then mustbe carefully protected from normal, volt ampere loads. The presentinvention, however, eliminates the need for this'careful protection.

The switch of FIGURES l, 2 and 3 is easily modified for double throwoperation by the addition of a second, stationary contact ill as shownin FIGURE 4 which increases the versatility of the relay within thescope of the present invention. As shown, reference numerals of theelements of the switch of FIGURES l, 2 and 3 are used here with a primeadded to indicate corresponding elements. The movable blade 12 isbifurcated into a power contact portion 14' and a signal cotact to. Thefirst set of stationary contacts 13 includes the power contact 2d andthe signal contact 22. A second set of stationary contacts 33 isprovided, including a power contact 2%" and a signal contact 22". Thepower contact Ztl is positioned in advance of the signal contact 22 andis nearer the opposite set of contacts 18. As before, a relay having anarmature 24 with a lever arm 26 connected to it moves the blade 12.

in the normal, deenergized state, the lever arm as is positionedslightly away from the movable blade 12, which is normally biased intocontact with the second so of stationary contacts 18. The second set ofcontacts may therefore be termed the normally closed (NC) set. On fullenergization of the relay, lever arm 26, bearing upon the signal contactportion of the 16 movable blade 12, drives the blade 12 into engagementwith the first set of stationary contacts 13, which may be termed thenormally open (NO) set. After energization, therefore, the state of theswitch is as described above in connection with the embodiment ofFIGURES l, 2 and 3.

t It will be recognized that the choice of resilient materials, theamount of normal spring bias, the relative dimensions of the power andsignal portions 14, 16' of the movable blade 12' are all parameterswhich may be varied to determine the contact pressures available and thetime interval between the breaking of a signal contact and the breakingof a power contact and the subsequent making of the opposite powercontact and the making of the opposite signal contact. Those skilled inthe art will readily determine the proper combination for optimumperformance when the lever arm 26' is released. A similar sequencefollows on deenergizing of the relay. normal bias of the movable blade12, and the added force onthe signal contact portion 16 breaks thesignal contact first and then the power contact. As the blade 12 movesto its rest position, the power contact portion makes first and thenormal bias of the blade causes the signal contact portion 16 to makewith the normally opened (NO) set of stationary contacts 18 also.

An alternative embodiment of the invention is shown in FIGURE 5 andincludes a bifurcated movable blade 112 of resilient base metal,cantilevered from a mounting (not shown) and which is normally biasedaway from a fixed stationary contact 118 also of a base metal. As above,one of the blade portions is a power contact portion 114 and the otheris a signal contact portion 116. The stationary contact 118 in thisembodiment is a pair of stepped or staggered contact surfaces on acommon, rigid member. A power contact surface 120 is positioned inadvance of a signal contact surface 122 with respect to the approachingmovable blade 112. Both contact elements are plated or flashed with anoble metal to increase the surface conductivity. 7

Attached to a relay armature 124 is a lever arm 126 which is shown inthe partially energized state, adjacent the one of the portions of thebifurcated blade 112 corresponding to the signal contact portion 11s. Asbefore, when fully deenergized, the lever arm rests apart from the bladeand when energized, the lever arm exerts by a force against the signalcontact portion of the blade, to move the entire blade against thestationary contact 118. The power contact portion 114 makes with thestationary power contact surface 12% and the lever arm 126 continues todrive the blade at the signal contact portion 116 until it, too, makeswith the signal contact surface 122 of the stationary member 118.

When the relay is deenergized and the lever arm 126 returns to its restposition, the internal forces of the resilient signal portion 116 of theblade 112 causes it to break first, quickly followed by the powerportion 114. The normal resilient bias of the movable blade maintainsthe sets of contacts in the open state. It will be readily appreciatedthat the spacing between the power and signal contact surfaces 129, 122is determined by many fac tors, including the resilience of the blademember 112 and the desired speed of make and break of the contacts. Asecond stationary contact set may be positioned to be normally closedwhen the relay is deenergized and therefore this embodiment, too, canfunction as a single pole, double throw switch, as shown in FIGURE 4above.

Still another configuration according to the present invention is shownin the embodiment ofFIGURE 6. The relay is substantially similar to thatof the earlier embodiments and employs a pair of stationary contacts 218comparable to those of the device of FIGURE 1. These stationary contacts218 are resilient and one of them, designated the power contact 220, is,as in the device of FIGURE 1 above, positioned in advance of the other,designated the signal contact 220, with respect to a movable blade 212.In this embodiment, a non-bifurcated movable blade212 is used and thedesired make-break sequence of operation is effected by the movement ofthe blade into engagement first, with the resilient power contact 22%which is elastically deformed until contact is made with the signalcontact 222 as well. A relay armature 224 carries a lever arm, 226 todrive the movable blade 212. When the relay is energized,

aces-3,335

6 the blade is driven as described above and deenergization of the relayreleases the armature 224 and the lever arm 226. The blade 212 springsaway from the contacts under the combined forces of its own normalresilient bias and the added spring force of the deformed stationarycontacts.

A second set of stationary contacts (not shown) can be added as in theembodiment of FIGURE 4 above, to enable single pole, double throw,operation with the contact sequence maintained as described above, andsuch modification is well within the skill of the art.

For certain applications, it is preferred to have a pair of so-calledcontact buttons as the contacting surfaces and these are carried by boththe moving blade and the stationary contact. Contact buttons can be madeof special materials of high conductivity or long wear or a combinationof these properties and may be plated or flashed without the necessityof plating or flashing the entire blade and stationary contact assembly.Depending upon the usage envisioned, the buttons can be planar orcurved. The curved buttons exert a much higher contact pressure since,ideally, touching spheres meet at a point. At the normal springpressures used in relay switches, the contact pressure at a point can besubstantial, resulting in some elastic deformation at the point ofcontact which materially aids in self-cleaning of the contact surfacesby fracturing and flaking off contaminant films.

One such switch embodiment 31% having contact buttons on a bifurcatedmovable blade 312 and on the sta tionary contacts 318 is shown in FIGURE7. Buttons 330, 332 of different thicknesses are mounted to the portionscorresponding to the power and signal portions 314, 316 of the movableblade 312. The stationary contact 318 consists of a pair of respectivelycorresponding buttons 324, 336 fastened to rigid mounts 32%, 322. Thepower contact button 334 is positioned in advance of the signal contact336 button as in earlier embodiments described above. The spacingbetween the buttons 324, 326 is partially determined by the relativethickness of the buttons and partially depends on the other parametersof the system, such as resilience of the blade 312 and relative size ofthe signal and power portions 314, 316. Upon energization of the relaythe farther extending or power contact button 334 is made first and theblade 312 is elastically deformed until contact is made with the signalbutton 336. A slight overtravel can be provided to assure positivecontact with both buttons and, upon release of the relay, the break ofthe signal contacts 332, 336 precedes the break of the power contacts330, 3345.

Other combinations of switches utilizing button contacts on resilientstationary contact members and a nonbifurcated blade will be obvious tothose skilled in the art. In all variations, through the physicalplacement of the components, the power contact always makes before thesignal contact and, on deenergization of the relay, the signal contactbreaks before the power contact.

It will also be recognized by those skilled in the art, that, in each ofthese embodiments, the power contact should be physically displaced withrespect to the signal contact to assure early making and late breakingof the power contact and, that at least one contact should be resilient.

It will also be readily apparent to those familiar with relay circuitdesign that, in every instance, a second set of stationary contacts,identical to the first, may be provided to cooperate with the movableblade to extend the versatility of the switch to include double throwoperation. In such configurations, the movable blade is normally biasedinto engagement wtih one pair of contacts, the normally closed set (NC)and, upon energization of the relay, the blade makes with the other,ornormally open (NO) pair.

Other configurations of the relay are also possible, such as thosewherein the armature is centrally mounted on a pivot to operate twomovable blades in a double-throw, double pole arrangement, firstconnecting one diagonally opposed set of contacts and then,alternatively, connecting the other diagonal set. Still other relayconfigurations provide a plurality of movable blades, which are drivenby a single relay armature, and it will be appreciated that every bladeand contact set can be arranged according to the principles of thepresent invention.

What is claimed as new is:

l. A switching device for providing a low impedance circuit between afirst and a second terminal to signals in the dry circuit range, saiddevice comprising: movable contacting means including coplanar first andsecond re- .silient members having a noble metal plating for increasedsur ace conductivity and being adapted to be commonly connected to thefirst terminal; stationary contacting means including a correspondingpair of contact members, positioned one in advance of the other withrespect .to said movable means, for mechanically and electricallyengaging said first and second resilient members, respectively, saidpair of members having a noble metal plating for increased surfaceconductivity and being adapted to be commonly connected to the secondterminal; and

r driving means energizable to contact said movable contact means tobring said movable contact means into engagement with said stationarycontact means in an ordered sequence, said driving means whendeenergized moving out of contact with said contact means to releasesaid movable contact means from engagement with said stationary contactmeans in the reversed sequence, whereby signals of voltage and currentsexceeding the dry circuit range are incapable of damaging at least saidother contact member and the resilient member cooperating therewith.

2. in a relay having first and second states, including a switch portionto interconnect elements of a low level circuit and having means formaintaining the low level capability of said switch after theapplication of higher level signals, said means comprising: a movableblade member of a resilient base metal having a noble metal plating toprovide a highly conductive contacting area, said blade being mountedfor cantilever operation and having a normal resilient bias; abifurcated stationary contact of base metal having first and secondcontact bifurcations commonly connected electrically, said contact beingpositioned in the path of movement of said blade member with said firstbifurcation being positioned in advance of said second bifurcationrelative to said blade member, said contact bifurcations having a noblemetal plating to provide a highly conductive contacting area; anddriving means operable in response to the first state of the relay toengage said blade for moving said blade member against the normal biasinto touching engagement with said first and second conductivebifurcations in order, said driving means being responsive to the secondstate of the relay to disengage said blade for releasing said blademember, said blade member under the force of the normal bias, firstdisengaging said second contact bifurcation to confine detrimentalelectrical activity to said first birfurcation on both make and break ofthe switch, whereby the low level signal conductivity of said secondbifurcation is maintained substantially unimpaired.

3. In a low level signal switching device, a combination for prolongingthe operating life of said device, the combination comprising: movableswitch means of a base metal having a noble metal flashing including afirst resilient meber having a first contact and a second resilientmember having a second contact, coplanar therewith; stationary contactmeans of a base metal having a noble metal flashing including a thirdcontact positioned in the path of said first contact and fourth contactpositioned in the path of said second contact in a staggered alignment,one of said contacts being positioned nearer said movable switch means;and driving means energizable for engaging first and second contacts tomove said contacts into electrical and mechanical contact with saidthird and fourth contacts, respectively, in sequential order, and saiddriving means when deenergized, disengaging said contacts to breakcontact in reverse sequential order, whereby the contact pair first toclose is the last to open, and whereby voltage differences and currentflow tending to destroy the noble metal flashing are confined to saidcontact pair first to close and last to open, thereby maintaining thelow level signal capabilities of the contact pair last to close andfirst to open.

4. In a relay switch including base metal contact elements and a noblemetal plating to provide highly conductive contact surfaces for lowlevel signals, a combination for maintaining the low level capability ofsaid switch after application of high level signals of magnitudessufficient to damage the noble metal plating at the area of surfacecontact, said combination comprising: first and second contact armsmovable relative to each other for conductive engagement, one of saidcontact arms being bifurcated at a free end; a first pair of contactsrespectively located on each bifurcation and facing the other of saidcontact arms; a second pair of contacts on the other of said contactarms, respectively positioned to conductively engage said first pair ofcontacts, one contact of one of said pairs being positioned in advanceof the other contact of said one of said pairs relative to said other ofsaid pairs; and means for contacting said arms to move said armsrelative to each other for establishing conduction between correspondingcontacts of said pairs in a predetermined order and for disengaging saidarms to break break conduction in reverse order, whereby at least oneset of corresponding contacts maintains a highly con ductive contactingsurface for conducting low level signals after application of high levelsignals across said first and second contact arms.

5. In a relay switch having base metal contact elements with a noblemetal plating to provide highly conductive contact surfaces for lowlevel signals in the dry circuit range, a combination for maintaininglow level circuit capability after application of signals of electricalmagnitude sufficient to damage the noble metal plating, the combinationcomprising: first and second contact memberst, movable relative to eachother having noble metal plated contact surfaces and respectivelyadapted to establish a low level circuit in the dry circuit range; apair of resilient contact projections extending from said first memberand having noble metal plated contact surfaces, one of said pair beingcloser to said second contact member than the other of said pair; andmeans contacting said members to impart movement to said contact membersfor first establishing conduction between said second member and saidone of said pair of projections and then establishing conduction betweensaid second member and said other of said pair of projections, wherebyapplication of signals of magnitude sufiicient to damage the noble metalcontact surface adversely affects only the contact area between saidsecond member and said one of said pair of contacting projections.

6. A switching device adapted for inclusion in a dry circuit formaintaining the dry circuit capability of said device after applicationof signals of magnitude greater than dry circuit signals, said switchingdevice comprising the combination of: moving switch means including abifurcated element of a base metal having a noble metal contact surfacecommonly connected to a first terminal with a pair of contact membersrespectively located on each bifurcation, said bifurcations beingresiliently coupled together for limited independent movement; firststationary contact means of a base metal having a noble metal contactsurface including a pair of first stationary contacts respectivelypositioned to make first with one of said pair and then the other ofsaid pair of contact members; second stationary contact means includinga pair of second stationary contacts respectively positioned oppositesaid first stationary contact means to make first with said one of saidpair and then said other of said pair of con tact members, said movingswitch means being normally biased into engagement with said firststationary contact means; and driving means energizable to overcome saidswitch means bias for disengaging said moving switch means from saidfirst stationary contact means to break the respective connection and toengage said moving switch means with said second stationary contactmeans for making the respective connections, said one of said pair ofcontact members always making first and breaking last with thecorresponding stationary contacts and the other of said pair of contactmembers always making last and breaking first with the correspondingstationary contacts, whereby damage caused by voltages and currents ofmagnitudes greater than the dry circuit range is confined to said one ofsaid pair of contact members, and said other of said pair of contactmembers maintain its dry circuit capabilities.

References Cited in the file of this patent UNITED STATES PATENTS1,387,796 Mason Aug. 16, 1921 1,958,685 Tevander May 15, 1934 2,068,093Terjesen Jan. 19, 1937 2,117,047 Wheelock May 10, 1938 2,285,210 KemptonJune 2, 1942 2,341,931 Lloyd Feb. 15, 1944 2,473,982 Wood June 21, 19492,507,381 Morse May 9, 1950

